The process industry employs process variable transmitters to monitor process variables associated with substances such as solids, slurries, liquids, vapors, and gasses in chemical, pulp, petroleum, pharmaceutical, food and other processing plants. A process temperature transmitter provides an output related to a sensed process temperature. The temperature transmitter output can be communicated over a process control loop to a control room, or the output can be communicated to another process device such that the process can be monitored and controlled.
In some applications, the temperature transmitter is mounted directly to the temperature sensor assembly containing the temperature sensor. In other applications, the temperature transmitter is mounted remotely from the temperature sensor assembly to protect the electronics of the transmitter from the environment surrounding the temperature sensor.
One type of temperature sensor is a thermocouple, which includes two conductors formed of different materials and connected at a junction referred to as the “hot” junction. Due to the Seebeck Effect, a voltage develops across the free ends of the conductors when a temperature gradient exists between the free ends and the hot junction.
The amount of voltage between the free ends of the thermocouple conductors is a function of the temperature differential between the free ends and the hot junction. As a result, the voltage between the free ends can be used to determine the temperature at the hot junction if the temperature at the free ends is known. The temperature at the free ends is referred to as the reference temperature.
Under the prior art, the free ends of the thermocouple conductors extend into the temperature transmitter where the voltage between the free ends is measured. A temperature sensor within the transmitter provides the reference temperature of the free ends. Using this reference temperature and the measured voltage, the temperature transmitter calculates the temperature at the hot junction. This construction, in which the thermocouple conductors extend all the way to the interior of the temperature transmitter, is used for both directly mounted transmitters and remotely mounted transmitters.
A second type of temperature sensor is a resistance temperature device (RTD). RTD sensors utilize the fact that the resistance of a conductor changes based on the temperature of the conductor. By measuring the resistance of the RTD, it is possible to look up the corresponding temperature associated with that resistance level. To measure the resistance, a current is passed through the RTD and a voltage across the RTD is measured.
There are two-wire, three-wire, and four-wire implementations of RTD sensors. In the two-wire implementation, one wire is connected to one end of the RTD and a second wire is connected to the other end of the RTD. Current is passed through the two wires by a current source or a voltage is applied across the two wires by a voltage source and the resulting voltage/current is measured. The resulting combination of current and voltage is then used to determine the resistance of the RTD. Such two-wire implementations are prone to error, however, because the wires leading to the RTD sensor have inherent resistances that affect the measured current/voltage near the voltage/current source.
To remove this parasitic resistance from the measurements, three-wire implementations use a bridge circuit that applies a current to the RTD using two of the three wires and senses a voltage on one end of the RTD using a third sense wire that does not have current passing through it. The bridge circuit is designed to counteract the parasitic resistances in the two current carrying wires. The voltage sense wire does not have current passing through it and as such, does not generate any parasitic resistance.
In the four-wire implementation, two wires are connected to two respective ends of the RTD sensor and provide current to the RTD sensor. Two additional voltage sense wires are also connected to the two respective ends of the RTD sensor. The two voltage sense wires do not have current passing through them and as such the voltage measured by the two voltage sense wires does not include parasitic resistances.